1. Field of the Invention
The present invention relates to a method of attenuating ground roll in seismic data signals. The method can produce ground roll attenuated seismic data signals that provide a clearer view of the underlying geologic structure. The method is particularly suited for use with seismic data acquisition systems having small single-sensor receiver arrays where ground roll cannot be attenuated by stacking.
2. Description of the Related Art
Seismic data signals are collected to remotely sense subsurface conditions. particularly in connection with the exploration for and production of hydrocarbons such as oil and natural gas. To gather the seismic data, acoustic sources such as explosives, airguns, or vibrators are typically used to produce an acoustic signal that is transmitted through the geologic formations. Changes in acoustic impedance between different geologic layers cause a portion of the acoustic energy to be reflected and returned toward the earth""s surface. These reflected signals are received by seismic sensors and are processed to create maps of the subsurface geology. A majority of seismic information regarding the subsurface is obtained using pressure wave data, where a pressure wave is produced by the acoustic source, reflected by interfaces between the earth""s subsurface layers, and then received by seismic sensors located on or near the earth""s surface.
A portion of the acoustic energy produced by the acoustic source is not, however, transmitted downward toward the subsurface formations, but instead travels horizontally along the earth""s surface. This portion of the seismic signal, referred to as ground roll, travels at the Rayleigh wave velocity, which is typically much slower than the velocity of the pressure wave described above. Although the pressure wave typically travels much faster than the ground roll wave, the pressure wave must travel a much greater distance from the acoustic source to the reflecting layer to the seismic sensor than the ground roll wave that is transmitted along the surface of the earth from the source to the sensor, and it is not uncommon for a pressure wave and a ground roll wave to arrive at a seismic sensor simultaneously. Because the ground roll wave typically contains no information regarding subsurface geologic structure being investigated, it must be attenuated (i.e. removed) to the greatest extent possible before the seismic data is used to produce maps of the subsurface. Ground roll is generally considered a dominant noise source and effective removal of the ground roll signal often greatly enhances the quality of the subsurface image obtained during the seismic survey.
Ground roll does not travel with a unique propagation velocity, but instead displays a wide velocity range that depends on the seismic signal frequency. The dispersive character of the ground roll is one reason for the relatively long time duration of the ground roll signal. This dispersive character also makes it more difficult to develop methods of effectively attenuating ground roll in seismic data signals.
A conventional approach to the problem of ground roll suppression in seismic data processing is to use receiver arrays during data acquisition, and then to stack together the seismic data signals obtained from each of the receivers. The use of receiver arrays has some distinct disadvantages, however, both from a geophysical point of view and from an economic point of view. Currently, seismic data acquisition systems typically employ receiver arrays whose spatial extent is such that noise waves with wavelengths up to 1.4 times the sensor pattern length are attenuated. This leads to a spatial smearing effect: the response at a particular receiver station is the sum of all individual sensors in the receiver array. There is also a trend in the industry towards smaller bin sizes. The standard 50xc3x9750 meter bin sizes will likely be reduced, for example to 40xc3x9740 meters or 30xc3x9730 meters, to overcome spatial aliasing problems and to increase resolution. As an example, high resolution is required for reservoir monitoring to establish 3D-impedance maps of the reservoir. This concept of smaller bin sizes is compromised by the spatial smearing effect introduced by conventional receiver arrays.
Apart from the smearing effect, sensor patterns are also ineffective because they are too short. The stack array approach (effectively resembling a very long receiver array), which is effective in attenuating ground roll, requires full fold geometry, is too expensive, and is not often used. An overlay of patterns also counteracts high resolution. Another trend in high-resolution seismics is the use of multi-component recording, for example by using a pressure wave source and recording all mode-converted waves or by using a shear wave source and recording the shear wave response. However, for shear waves, the patterns have to be short (less than approximately 12 meters) to avoid signal attenuation due to shear statics. This further compromises the effectiveness of the patterns with regard to ground roll attenuation, and thus creates a signal to noise problem.
A single-sensor small-array seismic data acquisition system can avoid many of these difficulties. In this type of system, the seismic responses from each of the individual sensors can be individually processed (i.e. the seismic data signals from each of the seismic sensors in an array at a particular receiver station are not immediately stacked to attenuate the ground roll present in the seismic data signals).
Several benefits are associated with single-sensor small-array acquisition systems. Deep shot holes are not required to reduce the amount of ground roll generated: shallow shot holes are sufficient. Shallow holes are less expensive to drill and they provide environmental benefits by not requiring deeper ground water reservoir zones to be penetrated. Shot (acoustic source) patterns are not required, which reduces the cost and complexity of the seismic signal generation process. Extensive geophone patterns are not required, thereby reducing equipment weight and cost and reducing the number of field employees required to perform a seismic survey. Because the seismic data signals from individual sensors are acquired (instead of immediately stacking the response from an entire sensor array), optimum data processing steps such as shotgather-based depth migration or amplitude versus offset analysis are possible. Subsequent data processing also inherently acts a perfect random noise attenuator, i.e. random noise is canceled without any additional cost or required processing. It may also be possible to reduce the actual sensor coverage at a particular receiver station, from 24 to 16 for instance, due to the improved signal to noise ratio of the single sensor data.
If a single-sensor small-array system is to be effective, however, a new method of attenuating ground roll in the received seismic data signals must be utilized.
It is therefore an object of the present invention to provide an improved method of attenuating ground roll in seismic data signals.
According to the present invention there is provided a method of attenuating ground roll in seismic data signals, the method comprising the steps of obtaining seismic data signals from nearby seismic sensors, creating a difference filter that incorporates an estimate of ground roll differential move-out between the seismic sensors, and applying the difference filter to the seismic data signals to produce a ground roll attenuated seismic data signal.
The inventive method can be used with one-pass and multiple-pass filter operations and can be used in connection with pairs of seismic data signals or with groups of seismic data signals from an areal pattern of nearby seismic sensors. Alternative embodiments of the method involve appropriately pre-processing the seismic data signals obtained from the nearby seismic sensors or involve deriving additional factors when creating the appropriate difference filter. The inventive method can be implemented in the time domain or in the frequency domain.